Crude oil washing is a system whereby oil tanks on a tanker are cleaned out between voyages not with water, but with crude oil – the cargo itself.  The solvent action of the crude oil makes the cleaning process far more effective than when water is used. There is usually a final water rinse but the amount of water involved is very low. The system helps prevent pollution of the seas from operational measures. COW is mandatory on new tankers under the International Convention for the Prevention of Pollution by Ships